1. Field of the Invention
The present invention relates to organic light emitting diodes (OLEDs). In particular, the present invention relates to a pulse mode OLED pixel or sub-pixel driver.
2. Description of Prior Art
An OLED device typically includes a stack of thin layers formed on a substrate. A light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between a cathode and an anode. The light-emitting layer may be selected from any of a multitude of fluorescent and phosphorescent organic solids. Any of the layers, and particularly the light-emitting layer, also referred to herein as the emissive layer or the organic emissive layer, may consist of multiple sublayers.
In a typical OLED display, either the cathode or the anode is transparent or semitransparent. The films may be formed by evaporation, spin casting, chemical self-assembly or any other appropriate polymer film-forming techniques. Thicknesses typically range from a few monolayers (i.e., a single, closely packed layer of atoms or molecules, perhaps as thin as one molecule), up to about 1000 to 2,000 angstroms.
Protection of an OLED display against oxygen and moisture can be achieved by encapsulation of the device. The encapsulation can be obtained by means of a single thin-film layer surrounding the OLED situated on the substrate.
High resolution active matrix displays may include millions of pixels and sub-pixels that are individually addressed by the drive electronics. The drive electronics for each sub-pixel can have several semiconductor transistors and other integrated circuit (IC) components. Each OLED may correspond to a pixel or a sub-pixel, and therefore these terms are used interchangeably hereinafter.
In an OLED device, one or more layers of semiconducting organic material are sandwiched between two electrodes. An electric current is applied across the device, causing negatively charged electrons to move into the organic material(s) from the cathode. Positive charges, typically referred to as holes, move in from the anode. The positive and negative charges meet in the center layers (i.e., the semiconducting organic material), combine, and produce photons. The wave-length—and consequently the color—of the photons depends on the electronic properties of the organic material in which the photons are generated.
The color of light emitted from the organic light emitting device can be controlled by the selection of the material used to form the emissive layer. White light may be produced by generating blue, red and green lights simultaneously. Other individual colors, different than red, green and blue, can be also used to produce in combination a white spectrum. The precise color of light emitted by a particular structure can be controlled both by selection of the organic material, as well as by selection of dopants in the organic emissive layers. Alternatively, filters of red, green or blue (or other colors), may be added on top of a white light emitting pixel. In further alternatives, white light emitting OLED pixels may be used in monochromatic displays.
Pixel drivers can be configured as either current sources or voltage sources to control the amount of light generated by the OLEDs in an active matrix display.
AMOLED displays are normally driven with constant luminance over a full frame cycle. A pixel is typically programmed once each frame period and the data is held constant by a storage capacitor (for analog pixels) or register (for digital pixels) until the next frame cycle when the pixel data is refreshed. This is known as a hold-type display in contrast to an impulse display like a cathode ray tube (CRT).